Television receivers and monitors (i.e., a television system not having a tuner) often include an "on-screen display" (OSD) system for displaying characters in one or more colors on the screen of an image display device such as a kinescope. The OSD system may, for example, be used to display the channel number of a currently tuned channel, status information or instructions.
Sometimes the OSD system has provisions for "blanking" the video image behind the characters in order to make the characters stand out in a field of video information. The characters may also be highlighted by "edging" the borders of the characters in black, by "shading" the characters on one side, or by displaying the characters within a black background box sometimes called a "surround". If the characters are to be highlighted in these ways, character signals having pulses corresponding to the blanking areas behind the characters and/or the black areas adjacent to the bodies of the characters, as well as character signals having pulses corresponding to the bodies of the characters themselves, must be generated and appropriate levels must be inserted into the video signals coupled to the display device at times corresponding to the character pulses and character blanking pulses.
In one well known type of OSD, one or more character signals representing the bodies of characters are coupled directly to respective ones of red, green and blue kinescope drive circuits. A character blanking signal for blanking the video image behind the characters and for "edging" the characters is coupled to an electronic video switch included in the path by which the luminance signal is coupled to the kinescope drive circuits. The switch operates in response to the pulses of the character blanking signal to insert a level corresponding to black in the luminance signal coupled to the kinescope drive circuits.
Many modern television receivers have a large portion of the intermediate frequency (IF), video and sound detection, and luminance, chrominance and deflection signal processing sections incorporated within a single integrated circuit (IC) sometimes called a "one-chip" or "jungle chip" IC. Such one-chip television ICs receive the IF signal produced by a tuner at an input terminal and generate either red, green and blue color (R. G, and B) output signals or luminance (Y) and red, green and blue color difference (R-Y, G-Y and B-Y) output signals at respective output terminals.
One-chip television ICs often also include OSD character signal input terminals. For example, the TA8680 one-chip television IC, available from the Toshiba Corp., has "red" and "green" OSD character signal input terminals intended to be used to receive red and green character signals for causing signal levels suitable for displaying red and green characters on the screen of a kinescope to be inserted into red and green color difference (R-Y and G-Y) output signals. The TA8680 IC also has a character brightness control input terminal intended to be used to receive an adjustable DC control signal for controlling the brightness of the red and green characters. Specifically, the level of the DC control signal determines a signal level which is inserted into the luminance (Y) output signal at times determined by the pulses of character signals coupled to the character signal input terminals.
While the use of a one-chip television IC such as the TA8680 is desirable since much of the signal processing portion of the television receiver is incorporated in a single IC, such ICs may limit the number of features that can be provided by the television receiver. For example, the TA8680 IC does not include input terminals for receiving character blanking signals by which video information can be blanked prior to the insertion of the signal levels corresponding to the pulses of the character signals into the R-Y and G-Y color difference signals. Therefore, ICs such as the TA8680 cannot be used by themselves to highlight characters by blanking behind the characters or by edging, shading or forming a surround.